Biology Reference
In-Depth Information
Cavefish (R)
Goldfish (R)
SY T
SY T
SYT
AFA
A
F
A
Cavefish (G101)
Cavefish (G103)
AFA
A
A
F
F
A
A
Gecko (G)
SYT
AFA
S YT
SY T
Chameleon (R)
Chicken (R)
SY T
A
F
A
Human (G)
Macaque (533)
A FA
A
F
A
A
YT
Human (R)
SY T
SY T
A
YT
A
YT
S YT
Macaque (563)
Marmoset (563)
SY T
AY T
Marmoset (556)
Marmoset (543)
A
YT
A
YT
A
A
A
YT
Y
YT
A
Goat (R)
Figure 7.2.
Phylogenetic relationships between vertebrate opsins (after Yokoyama, 1997)
showing amino acids inferred at sites 180, 277, and 285. The bold and outlined letters
refer to red pigment and green pigment characteristic amino acids respectively. Probable
amino acid substitutions at different evolutionary stages are boxed.
of extant tetrapods was acquired by a further amino acid substitution, Ala180Ser
(
Figure 7.2
).
Old World monkeys have both red and green pigments and therefore possess
trichromatic vision. Phylogenetic analysis of the primate opsins (Nei
et al
., 1997)
has indicated that the common ancestor of hominoids, Old World monkeys and
New World monkeys possessed a red visual pigment with AYT at the three criti-
cal sites (
Figure 7.2
) but no green pigment. The ancestor of the green pigment
gene is thought to have arisen by gene duplication ~35 Myrs ago in the Old
World monkey lineage (Nathans
et al
., 1986) and the green pigment was then
derived in two distinct steps (AFA
AYT
AFA;
Figure 7.2
).
New World monkeys have only one X-linked opsin gene but exhibit multiple
alleles at this locus (Jacobs
et al
., 1996; Neitz
et al
., 1991). Thus, in the squirrel
monkey (
Saimiri sciureus
), the visual pigments derived from three alternative alle-
les have λ max values of 532, 547, and 561 nm and manifest amino acids AFA,
AFT, and SYT respectively at the critical sites (Neitz
et al
., 1991). Whilst all male
and homozygous female squirrel monkeys are dichromatic, heterozygous females
that happen to have pigments with λ max values of 532 nm and 561 nm possess
trichromatic vision. Tovee (1994) suggested that color vision in New World mon-
keys might be an adaptation to allow a wide variety of color vision types within a
single family group. However, an alternative explanation, which does not invoke
group selection, would be for the alternative alleles to be maintained in the popu-
lation by overdominant selection. This postulate appears to be supported by the
independent evolution of triallelic systems in several other New World monkey
lineages represented by the marmoset (
Callithrix jacchus
), the saki monkey
